I am using a secondary email client (Thunderbird) my principle email client is Comcast. I keep getting blacklisted (re: spamming) in my secondary email account only. I do not receive or send spam!
What can I do?
Who is blacklisting you? It sounds like maybe your email account has been compromised and someone is using it to send spam. I would suggest going into the Account Settings for that email account and changing the password to a new strong password.
Delete the Comcast account email and set it up again under a new email address.